This invention relates to a photographic lens, more particularly a telephoto lens free from a secondary spectrum.
In telephoto lenses, even when chromatic aberration has been corrected with respect to light rays of specific two wave lengths, chromatic aberration remaining in connection with light rays of other wave lengths, namely the so-called secondary spectrum is markedly large as compared with that of a lens having a shorter focal length, and it is known that amounts of this secondary spectrum may determine the performance of telephoto lenses.
The amount of the secondary spectrum is usually about 1/500 to 1/1000 of the focal length, so that the longer the focal length is the more the color flare or dispersion becomes.
As for methods for decreasing the amount of secondary spectrum, it has been proposed to use a positive lens made of anomalous dispersion optical glass materials in the front group of a telephoto lens, and for such optical materials, optical crystals of fluorite etc. and anomalous dispersion glasses having a low refractive index and a low dispersion rate are known.
Further it is also effective for dimishing the secondary spectrum to use lanthanum glass, instead of flint glass or dense flint glass for the negative lens in the front group of a telephoto lens. U.S. Pat. No. 3,868,174 discloses a telephoto lens in which the first positive lens in the front group is made of fluorite and the negative lens is made of lanthanum glass.
Meanwhile, a telephoto lens of rear focus type in which part of the rear group elements is moved to effect focusing is known in Laid-Open Patent Specification No. Sho 50-139732 (U.S. Pat. No. 3,854,797). It has been found, however, that the secondary spectrum cannot always be diminished by use of an optical material having an anomalous dispersion characteristics in a lens having a power arrangement and lens shape suitable for the rear focus as disclosed by the prior art. Therefore, the favorable effects of the low refractive index and the low dispersion rate cannot be fully utilized if the method of using the material is not appropriate.